[GHC] #14462: deriving on associated data types fails to find constraints

14 Nov 2017

      #14462: deriving on associated data types fails to find constraints
-------------------------------------+-------------------------------------
           Reporter:  mf825          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  associated types, deriving         |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE TypeFamilies, UndecidableInstances #-}

 class D a where data DT a
 class C a where data CT a

 instance (D a, Eq (DT a)) => C (Maybe a) where
   data CT (Maybe a) = CTMaybe (DT a) deriving (Eq)

 {-
 $ stack --resolver=nightly-2017-10-20 exec -- ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.2.1

 $ stack --resolver=nightly-2017-10-20 exec -- ghci Main.hs
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 (0.00 secs, 0 bytes)
 Loaded GHCi configuration from /home/mf/.ghci
 [1 of 1] Compiling Main             ( Main.hs, interpreted )

 Main.hs:7:48: error:
     • No instance for (Eq (DT a))
         arising from the first field of ‘CTMaybe’ (type ‘DT a’)
       Possible fix:
         use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
     • When deriving the instance for (Eq (CT (Maybe a)))
   |
 7 |   data CT (Maybe a) = CTMaybe (DT a) deriving (Eq)
   |                                                ^^
 Failed, 0 modules loaded.
 (0.03 secs,)
 Prelude>

 -- if i remove the offending @deriving@ clause above and add this line,
 everything is fine.
 -- use -XFlexibleInstances -XStandaloneDeriving for this.
 deriving instance Eq (DT a) => Eq (CT (Maybe a))

 -}
 }}}

 checked on linux with ghc8.0.2 and 8.2.1.  thanks!  and sorry if i've
 missed a previous report covering this.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14462
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